Helion Energy accelerates two small, compact balls of plasma into one another at a speed of hundreds of kilometres a second. The conditions created by the collision should, in theory, be sufficient to force the nuclei together, heat them and ignite fusion. This method has some notable advantages. Although magnetic fields are still used to confine the plasmas, the arrangement is far less elaborate than a tokamak, so the device can be a lot smaller. The reaction is intense and is over in a fraction of a second, and neutrons are only produced at the point where the plasmas collide, making it easy to collect them to breed tritium.

Helion researchers have used the technique to smash two plasmas together and achieve a temperature of 25 million degrees. A device that is three times larger could achieve ignition and breakeven.

50 Helion Energy Fusion engines could transmute the US stockpile of nuclear waste.

2. The Canadian firm General Fusion based in Burnaby, British Columbia, is using a method called magnetised plasma fusion.

According to company spokesman Michael Delage, the first laboratory tests of the design have gone well, achieving a temperature of 5 million degrees for 1 microsecond. It remains to be seen whether this approach can be scaled up all the way to fusion - and beyond that to break-even. "There are no magnetised plasma experiments that we are aware of at the plasma temperatures and densities necessary for net-gain fusion," Delage says. "The only way to verify this is by experiment." The firm has raised the $30 million it says it needs, some of it from Amazon founder Jeff Bezos.

3.

Tri Alpha Energy, a secretive California-based company, is believed to have raised $90 million for its variant of the field-reversed technique; among its investors is Microsoft co-founder Paul Allen. In a rare public communication a year ago, Tri Alpha researchers showed how they had collided two plasma balls at a temperature over 5 million degrees and held them together for up to 2 milliseconds (Physical Review Letters, vol 105, p 045003). Tri Alpha says it will produce a working commercial reactor some time between 2015 and 2020 - possibly before ITER fires up for the first time.